1. Field of the Invention
The present invention generally relates to a method and system for thermally treating a magnetic layer, and more particularly to a method and system for fast and local annealing of a magnetic stack.
2. Description of the Related Art
Magnetic random access memory (MRAM) devices typically must undergo a thermal treatment to set some of the magnetic layers in a desired orientation. Generally, the samples must be held (e.g., for about 1 hour) at relatively high temperatures (300-400° C.), while a large (1-2 Teslas) and uniform magnetic field is applied.
However, a practical difficulty arises for performing such steps on large (200 or 300 mm diameter) wafers, as this entails building large anneal ovens and magnets to achieve uniform conditions over the wafer.
Additionally, the process is slow because of the size of the ovens, and the set direction cannot be varied from point-to-point on the wafer.
In a conventional technique 500, as shown in the flowchart of FIG. 5, a batch process uses a combined magnet and furnace, either in vacuum or inert gas.
In this conventional technique, the process flow 500 includes first loading a batch of wafers into a relatively large anneal oven and pumping down the oven (step 510) (optionally ramping up the magnetic field, if an electromagnet is being employed), ramping-up the temperature (e.g., 300-400° C.) and holding the temperature at a target value (step 520), applying a magnetic field and ramping down the temperature (step 530) (optionally ramping down magnetic field, if electromagnet was employed), and finally unloading the batch (step 540).
However, the above-described conventional technique 500 has many disadvantages.
First, there is uneven field and temperature uniformity across the batch. Additionally, there are a long ramp-up and ramp-down (e.g., of the temperature in the oven) times due to the large heat capacity. This makes the entire process relatively slow.
Additionally, since a batch mode processing is employed, one must wait for a batch of wafers to exist, in order to process them efficiently. That is, single wafers are generally not processed due to cost constraints. Instead, the user must work in increments (e.g., or batches), as such increments/batches are accumulated. Thus, there is little flexibility in the processing and difficult to tailor the processing to numbers of wafers less than a batch. Hence, in the conventional techniques, there is no local option in processing wafers.
Another disadvantage of the conventional techniques is the space required by such systems. That is, such systems are generally very large, heavy and cumbersome.
Additionally, complex and expensive magnetic field units are required, as good uniformity across the stack is normally needed. Without such complex and expensive magnetic field units, such uniformity is seldom achieved.